An integrated drive generator (IDG) of an aircraft engine is a hydraulicly controlled electrical generator which is mounted to the aircraft engine and driven by the engine. The IDG is designed to operate successfully at any aircraft attitude. It is also designed to be internally sealed from ambient atmosphere during operation. The IDG is filled to a predetermined level with a liquid which is used, for example, to hydraulically vary the transmission ratio of a variable ratio transmission from the aircraft engine to the generator and also to lubricate and cool components of the IDG. If the IDG is over-filled with liquid, excessive liquid thrashing losses and heat generation will occur during operation which can damage the IDG components. If there is too little liquid in the IDG, damage to the IDG components can result because of overheating and inadequate lubrication, and proper hydraulic control of the IDG components may not be possible. Thus, it is desirable to be able to monitor the liquid volume in the IDG.
Known methods and apparatus for determining the volume of liquid in an IDG are dependent on the liquid level in the IDG. A dip stick is an example of such a known apparatus. These are problematical because the measurements obtained with level dependent techniques will not be accurate for a volume of irregular shape where there are changes in the liquid level due to changes in attitude, temperature and during acceleration. There is a need for an improved apparatus and method for determining the volume of liquid in an IDG liquid reservoir of irregular shape which provides accurate volume indications even when the aircraft is operating in various attitudes, temperatures and accelerations up to 60 g.
It is known from U.S. Pat. Nos. 2,162,180 and 4,553,431, for example, to determine the amount of liquid in a container using the gas low (i.e., PV=constant) by providing a special arrangement for injecting a fluid, especially a gas, into the gas in the ullage volume of the container and monitoring the resulting pressure change to determine the ullage volume and, then, by substracting the ullage volume from the total container volume to determine the liquid volume in the container. However, these known apparatus and methods are disadvantageous in that they require special, additional arrangements for injecting a known sufficiently large volume of a fluid into the container. For example, in U.S. Pat. No. 4,553,431 an air pump is provided for injecting air into the ullage gas volume to change the pressure for determining the quantity of liquid in the container. Air is also injected in the apparatus and method of U.S. Pat. No. 2,162,180 but the patentee states that a known volume of a liquid could be injected for measuring purposes.